The present invention relates generally to electrical stress control in electrical power cables, and more particularly to an article and method for controlling electrical stress in a region of high electric field strength associated with electrical power cables and their associated accessories.
As used herein, “high voltage” generally refers to voltages sufficiently high to cause breakdown of the cable insulation at cable shield discontinuities. Without limiting the scope of the present invention, in some implementations, “high voltage” generally refers to voltages of 50 kV or greater, although the present invention is also beneficially used with lower voltages.
A typical high voltage cable includes a central electrical conductor, a semiconducting layer (also referred to herein as a conductor shield) surrounding the electrical conductor, an electrically insulating layer covering the conductor shield, and a semiconducting layer (also referred to herein as an insulation shield) over the insulating layer. In terminating such a cable, it is customary to remove or cut back each successive layer of the cable to expose the layer below. Cutting back the semi-conductive cable shields causes a discontinuity in the cable electric field, resulting in high electric stress at the cut ends of the shields. The high electrical stress can cause electrical discharges to occur, which in turn tends to cause breakdown of the insulating layer of the cable.
The thickness of the cable insulating layer is dependent upon the cable voltage class, with higher voltage cables having a thicker insulating layer. Often, the thickness of the insulating layer could be reduced if the insulation material is made of higher quality (i.e., higher purity). For example, in the United States the insulation thickness of a 69 kV class cable is about 650 mils. A similar cable in Europe, the 72 kV class cable, has an insulation thickness ranging from 400 mils to 470 mils. The reduced insulation thickness provides benefits such as reduced cable size, weight and cost resulting from a decrease in the amount of insulating material used.
Although benefits are provided by a reduced insulating layer thickness, the reduced insulation thickness also forces cable accessories, such as cable terminations, to withstand higher electrical stress at cable shield discontinuities. Unless properly accounted for, the additional electrical stress may lead to failure of the cable and/or cable accessories. In some instances, the additional electrical stress is accommodated by substituting a cable accessory intended for a higher voltage class cable (e.g., using a cable accessory rated for 138 kV with a 110 kV cable having reduced insulation thickness). Although such accessory substitutions work, the cost differential of the higher-rated accessory is often significant. Accordingly, an arrangement that allows the use of cables having reduced insulation thickness with existing cable accessories in the same voltage class is desirable.